If you receive annuity payments from a nonqualified retirement plan, you must use the General Rule. Under the General Rule, you figure the taxable and tax-free parts of your annuity payments using life expectancy tables that the IRS issues. For a fee, the IRS will figure the tax-free part of your annuity payments for you. For more information, refer to Publication 939, General Rule for Pensions and Annuities.
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You would typically only use the General Rule for calculating that taxable amount of these distributions if the payer does not provide an accurate taxable amount on box 2a of the Form 1099-R. If the payer provided an amount in box 2a and did not mark box 2b Taxable amount not determined, in all likelihood the amount in box 2a is the correct taxable amount.